Weird sound when starting

Yesterday when i started it, it made a weird sound. It sounds like a tuned up indicator or what. It kinda raises the engine. Then i went to the mechanic and he said that i need to drain the transmission oil and fill it again. I have just used this van for only a month. So we did, but it had the same sound when i start it again. The same mechanic did the same oil change with another Serena, and the same sound i had finished with that vehicle. I wonder how come it worked with the same car, same problem as i have, but didn't work for me.